Ventricular fibrillation, an emergency that requires immediate medical attention, causes the person to collapse within seconds. It's the most frequent cause of sudden cardiac death. Emergency treatment includes cardiopulmonary resuscitation (CPR) and shocks to the heart with a device called a defibrillator.

Treatments for those at risk of ventricular fibrillation include medications and implantable devices that can restore a normal heart rhythm.

Loss of consciousness is the most common sign of ventricular fibrillation.

Early signs and symptoms

A condition in which the lower chambers of your heart beat too fast (ventricular tachycardia) can lead to ventricular fibrillation. Signs and symptoms of ventricular tachycardia include:

Chest pain

Rapid heartbeat (tachycardia)

Dizziness

Nausea

Shortness of breath

Loss of consciousness

When to see a doctor

If you or someone else is having the above signs and symptoms, seek emergency medical help immediately. Follow these steps:

Call 911 or the emergency number in your area.

If the person is unconscious, check for a pulse.

If no pulse, begin CPR to help maintain blood flow to the organs until an electrical shock (defibrillation) can be given. Push hard and fast on the person's chest — about 100 compressions a minute. It's not necessary to check the person's airway or deliver rescue breaths unless you've been trained in CPR.

Portable automated external defibrillators (AEDs), which can deliver an electric shock that may restart heartbeats, are available in an increasing number of places, such as in airplanes, police cars and shopping malls. They can even be purchased for your home. Portable defibrillators come with built-in instructions for their use. They're programmed to deliver a shock only when it's needed.

To understand ventricular fibrillation, consider a normal heartbeat.

What's a normal heartbeat?

When your heart beats, the electrical impulses that cause it to contract follow a precise pathway through your heart. Interruption in these impulses can cause an irregular heartbeat (arrhythmia).

Your heart is divided into four chambers. The chambers on each half of your heart form two adjoining pumps, with an upper chamber (atrium) and a lower chamber (ventricle).

During a heartbeat, the smaller, less muscular atria contract and fill the relaxed ventricles with blood. This contraction starts after the sinus node — a small group of cells in your right atrium — sends an electrical impulse causing your right and left atria to contract.

The impulse then travels to the center of your heart, to the atrioventricular node, which lies on the pathway between your atria and your ventricles. From here, the impulse exits the atrioventricular node and travels through your ventricles, causing them to contract and pump blood throughout your body.

What causes ventricular fibrillation?

The cause of ventricular fibrillation isn't always known. The most common cause is a problem in the electrical impulses traveling through your heart after a first heart attack or problems resulting from a scar in your heart's muscle tissue from a previous heart attack.

Some cases of ventricular fibrillation begin as a rapid heartbeat called ventricular tachycardia (VT). This fast, regular beating of the heart is caused by abnormal electrical impulses that start in the ventricles.

Most VT occurs in people with a heart-related problem, such as scars or damage from a heart attack. Sometimes VT can last less than 30 seconds (nonsustained) and may not cause symptoms. But VT may be a sign of more-serious heart problems. If VT lasts more than 30 seconds, it will usually lead to palpitations, dizziness or fainting. Untreated VT will often lead to ventricular fibrillation.

Most cases of ventricular fibrillation are linked to some form of heart disease.

Factors that may increase your risk of ventricular fibrillation include:

A previous episode of ventricular fibrillation

A previous heart attack

A heart defect you're born with (congenital heart disease)

Heart muscle disease (cardiomyopathy)

Injuries that cause damage to the heart muscle, such as electrocution

Use of illegal drugs, such as cocaine or methamphetamine

Electrolyte abnormalities, such as with potassium or magnesium

Ventricular fibrillation is always diagnosed in an emergency situation. Your doctors will know if you're in ventricular fibrillation based on results from:

Heart monitoring. A heart monitor that will read the electrical impulses that make your heart beat will show that your heart is beating erratically or not at all.

Pulse check. In ventricular fibrillation, there will be no pulse.

Tests to diagnose the cause of ventricular fibrillation

To find out what caused your ventricular fibrillation, you'll have additional tests, which can include:

Electrocardiogram (ECG). This test records the electrical activity of your heart via electrodes attached to your skin. Impulses are recorded as waves displayed on a monitor or printed on paper. Because injured heart muscle doesn't conduct electrical impulses normally, the ECG may show that a heart attack has occurred or is in progress.

Blood tests. Emergency room doctors take samples of your blood to test for the presence of certain heart enzymes that leak into your blood if your heart has been damaged by a heart attack.

Chest X-ray. An X-ray image of your chest allows your doctor to check the size and shape of your heart and its blood vessels.

Echocardiogram. This test uses sound waves to produce an image of your heart. During an echocardiogram, sound waves are directed at your heart from a transducer, a wand-like device, held on your chest. Processed electronically, the sound waves provide video images of your heart.

Coronary catheterization (angiogram). To determine if your coronary arteries are narrowed or blocked, a liquid dye is injected through a long, thin tube (catheter) that's fed through an artery, usually in your leg, to the arteries in your heart. The dye makes your arteries become visible on X-ray, revealing areas of blockage.

Cardiac computerized tomography (CT) or magnetic resonance imaging (MRI). Although more commonly used to check for heart failure, these tests can diagnose other heart problems. You lie on a table inside a doughnut-shaped machine. An X-ray tube inside the machine rotates around your body and collects images of your heart and chest.

For a cardiac MRI, you lie on a table inside a long tube-like machine that produces a magnetic field that aligns atomic particles in some of your cells. Radio waves aimed at these aligned particles produce signals that create images of your heart.

Emergency treatments for ventricular fibrillation focus on restoring blood flow through your body as quickly as possible to prevent damage to your brain and other organs. After blood flow is restored through your heart, if necessary, you'll have treatment options to help prevent future episodes of ventricular fibrillation.

Emergency treatments

Cardiopulmonary resuscitation (CPR). This treatment can help maintain blood flow through the body by mimicking the pumping motion your heart makes. CPR can be performed by anyone, including family members.

In a medical emergency, first call for emergency medical help, then start CPR by pushing hard and fast on the person's chest — about 100 compressions a minute. Allow the chest to rise completely between compressions. Unless you're trained in CPR, don't worry about breathing into the person's mouth. Keep up chest compressions until a portable defibrillator is available or emergency personnel arrive.

Defibrillation. The delivery of an electrical shock through the chest wall to the heart momentarily stops the heart and the chaotic rhythm. This often allows the normal heart rhythm to resume.

If a public-use defibrillator is available, anyone can administer it. Most public-use defibrillators voice instructions as you use them. Public-use defibrillators are programmed to recognize ventricular fibrillation and send a shock only when needed.

Treatments to prevent future episodes

If your doctor finds that your ventricular fibrillation is caused by a change in the structure of your heart, such as scarred tissue from a heart attack, he or she may recommend that you take medications or have a medical procedure performed to reduce your risk of future ventricular fibrillation. Treatment options can include:

Medications. Doctors use various anti-arrhythmic drugs for emergency or long-term treatment of ventricular fibrillation. A class of medications called beta blockers is commonly used in people at risk of ventricular fibrillation or sudden cardiac arrest.

Implantable cardioverter-defibrillator (ICD). After your condition stabilizes, your doctor is likely to recommend implantation of an ICD. An ICD is a battery-powered unit that's implanted near your left collarbone. One or more electrode-tipped wires from the ICD run through veins to your heart.

The ICD constantly monitors your heart rhythm. If it detects a rhythm that's too slow, it sends an electrical signal that paces your heart as a pacemaker would. If it detects ventricular tachycardia or ventricular fibrillation, it sends out low- or high-energy shocks to reset your heart to a normal rhythm. An ICD is more effective than drugs for preventing a fatal arrhythmia.

Coronary angioplasty and stent placement. This procedure is for the treatment of severe coronary artery disease. It opens blocked coronary arteries, letting blood flow more freely to your heart. If your ventricular fibrillation was caused by a heart attack, this procedure may reduce your risk of future episodes of ventricular fibrillation.

Doctors insert a long, thin tube (catheter) that's passed through an artery, usually in your leg, to a blocked artery in your heart. This catheter is equipped with a special balloon tip that briefly inflates to open up a blocked coronary artery. At the same time, a metal mesh stent may be inserted into the artery to keep it open long term, restoring blood flow to your heart. Coronary angioplasty may be done at the same time as a coronary catheterization (angiogram), a procedure that doctors do first to locate narrowed arteries to the heart.

Coronary bypass surgery. Another procedure to improve blood flow is coronary bypass surgery. Bypass surgery involves sewing veins or arteries in place at a site beyond a blocked or narrowed coronary artery (bypassing the narrowed section), restoring blood flow to your heart. This may improve the blood supply to your heart and reduce your risk of ventricular fibrillation.

Reprint Permissions

A single copy of these materials may be reprinted for noncommercial personal use only. "Mayo," "Mayo Clinic," "MayoClinic.org," "Mayo Clinic Healthy Living," and the triple-shield Mayo Clinic logo are trademarks of Mayo Foundation for Medical Education and Research.